Portugal has laid the blame for a massive power blackout on Spain after cuts sparked chaos across the Iberian Peninsula. One of Europe’s severest blackouts grounded flights, paralysed public transport, disrupted mobile communications and shut down ATMs on Monday. Portuguese Prime Minister Luis Montenegro speculated the source of the blackout was "probably in Spain". His country's national grid operator, REN, blamed a "fault" in Spain's electricity grid.

Spain's Prime Minister Pedro Sánchez said his government's priorities were to restored the country's electrical system and find the causes of the blackout so a similar event "never takes place again". More than 99% of energy demand in Spain had been restored by early this morning, according to the country's electricity operator Red Eléctrica. REN said all 89 power substations were back online and power had been restored to all 6.4 million customers. As life began to return to normal, authorities in Spain have yet to provide further explanations as to what caused the blackout. In just five seconds, a nation of 49 million people lost 15 gigawatts — equivalent to 60% of its national demand.

Eduardo Prieto, director of services for system operations at Spain's electricity operator, noted two steep, back-to-back "disconnection events" before the blackout. Speaking at a press conference on Tuesday, he said more investigation was needed to understand why they took place.

Spain’s meteorological agency, AEMET, said it hadn't detected any "unusual meteorological or atmospheric phenomena" on Monday and no sudden temperature fluctuations were recorded at their weather stations.

Portugal’s National Cybersecurity Centre dismissed speculation about foul play, saying there was no sign the blackout resulted from a cyber attack.

European Council President Antonio Costa said there were "no indications of any cyber attack" while Teresa Ribera, an executive vice president of the European Commission, also ruled out sabotage. Nonetheless, she said the blackout was one of the most serious episodes recorded in Europe in recent times.

Emergency workers in Spain said they rescued some 35,000 passengers stranded on railways and underground, with the blackout turning sports centres, train stations and airports into makeshift overnight shelters.

At Spain's largest train stations, scores of travellers waited this morning to board trains or to rebook tickets for journeys which were cancelled or disrupted.

At Madrid's Atocha station, hundreds of people stood near screens waiting for updates. Many had spent the night at the station, wrapped in blankets provided by the Red Cross. There were similar scenes at Barcelona's Sants station.

Airports in both countries also saw disruption yesterday, with thousands of travellers from the UK still trying to return today. Twenty-six flights from Portugal to the UK were cancelled, according to aviation analytics company Cirium.

A further 13 were axed from Spain to the UK. Airlines affected included British Airways, easyJet, Ryanair and TAP. Many other flights were delayed.